Many substances that act in trace amounts cannot be detected by conventional detection methods based on immunochromatographic methods. Recently, the development of rapid, convenient, and highly sensitive methods for detecting such substances has become essential. An immunochromatographic method with higher sensitivity than conventionally known immunochromatographic methods is required. Hence, various amplification techniques have been developed.
For example, a method that has been proposed over years involves directly or indirectly labeling aggregates formed of a specific binder and a binding substance with a tiny metal particles and particularly, with gold particles, so as to detect the aggregates. Metal particles possess advantages in that they allow direct visual inspection and signals to be generated are permanent and do not rapidly deteriorate. Hence, such metal particles have generated interest as labeling substances for simple and rapid tests. Furthermore, metal labels, and preferably, gold labels, have extremely low hygiene risks associated with operations that use them, so that they are more preferable than radioisotope labels. A general method involves labeling an antibody with a colloidal gold label and then blocking with a protein and a polymer the non-labeled portions to which no antibodies have adhered. Currently, a colloidal gold label amplification technique has been disclosed that uses a water-soluble polymer (acetal-PEG-SH, molecular weight of 10000) having a functional group capable of binding to metal particle surfaces and a reaction group capable of covalently binding to antibodies at the same time (JP Patent Publication (Kokai) No. 2005-214907 A). Moreover, a technique has been reported by which nonspecific adsorption of protein is suppressed by forming a polymer brush layer on the surface of gold using mercapto polyethylene glycols (HS-PEGs) with different molecular weights in a surface Plasmon (SPR) sensor (Katsumi Uchida. et al. Anal. Chem. 2005, 77, 1075-1080). However, it has never been reported that in an immunochromatographic method a metal particle label had been labeled with an antibody and then HS-PEGs having different molecular weights were used to improve detection sensitivity.